Problem: $84$ is what percent of $400$ ?
Solution: Having a percent of something means that you get that percent out of every $100$ We can set up a proportion to find out what percent of $400$ we need to take to get $84$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{\text{part}}}{{\text{whole}}}$ Which things do we know, and what are we trying to find? We are trying to find the ${\text{percent}}$ . Is $400$ the ${\text{part}}$ or the ${\text{whole}}$ The $400$ is the ${\text{whole}}$ . This means the ${\text{part}}$ is $84$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{84}}{{400}}$ If we divide the denominator of the fraction on the right by $4$ , it will be the same denominator of the fraction on the left. To keep things equal, let's also divide the numerator on the right by $4$ $ \dfrac{{\text{percent}}}{100 } = \dfrac{{84 \div 4}}{{400 \div 4}}$ $ \dfrac{{\text{percent}}}{100 } = \dfrac{{21}}{{100}}$ $ {\text{percent}} = {21}$ So $84$ is $21\%$ of $400$.